1. Field of the Invention
The present invention relates to a radio frequency identification (RFID) technology, and more particularly, to schemes of RFID interrogators.
2. Description of the Prior Art
Radio frequency identification (RFID) technology has already been widely applied in many fields such as aviation baggage monitoring, warehouse management, logistics management, burglarproof security service systems, automatic charge systems, quality management, and medical management, etc. due to improvements in technology and decreasing costs. In an RFID system, an RFID interrogator/reader and an RFID tag utilize wireless radio frequency (RF) signals to perform non-contact half-duplex data transmission between each other in order to achieve the purpose of object identification or data exchange.
Electronic tags can be divided into two schemes of active electronic tags and passive electronic tags. The active electronic tag includes an internal power source, and thus is able to actively send data to the RFID interrogator. In addition, the active electronic tag has a longer effective response distance, but has a shorter lifetime, a bigger volume, and a higher cost. The passive electronic tag does not have an internal power source, and it receives the RF signals sent from the RFID interrogator and converts the RF signals into the electrical energy required by the internal circuit operation. In comparison with the active electronic tag, the passive electronic tag has a smaller volume, a lower cost, and a longer lifetime.
When the electronic tag receives a query command sent from the RFID interrogator, the electronic tag will send a unique identification data back to the RFID interrogator for identification. The conventional passive electronic tag does not have an internal crystal oscillator, and the conventional passive electronic tag usually utilizes an LC oscillator to provide the required reference clock. However, the frequency resonated by the LC oscillator often drifts, and results in great variation in the symbol times of the signals sent back from the passive electronic tag. This condition will increase the possibility of errors occurring when the conventional RFID interrogator demodulates the received identification data, resulting in the integral efficiency of the RFID interrogator being reduced.